This invention relates to improvements over the structure of reflex-reflecting sheeting of a type having on the surface of the sheeting many isolated small compartments which are respectively hermetically sealed in the form of cells.
First of all, the background of the invention will be described.
A reflex-reflecting sheeting which retroreflects the incident light in the direction of incidence is widely used for various purposes including traffic signs and automobile number plates.
One of factors determining desirable reflex-reflecting properties of the reflex-reflecting sheeting is angle characteristic. As the angle of incidence to the surface of the sheeting increases, the amount of reflex-reflecting light against the amount of the incident light tends to decrease. It is desirable in the reflex-reflecting sheeting that the rate of this decrease in the reflex-reflecting light should be small, that is, the angle characteristic should be good.
In the reflex-reflecting sheeting of a type shown in FIG. 1 which has been used for a long time in the past, incident lights a, b incident upon glass beads 2 which are in contact with a transparent protective, film 1 pass through the glass beads 2 and a transparent focus layer 3 coated in a certain thickness under the glass beads 2 and strike upon and are retroreflected by a metallized layer 4 supported by a support film 5 underlying the focus layer 3.
This transparent focus layer 3 should desirably be coated in a uniform thickness such that it will form a partially spherical contour concentric with the glass beads. If, for example, the thickness of the focus layer 3 in the direction in which the incident light a having the angle of incidence of -4.degree. C. is incident is such that the incident light focuses upon the metallized layer 4 whereas the thickness of the layer 3 in the direction in which the incident light b having the angle of incidence of 30.degree. is incident is greater than that, the incident light b focuses at a point inside the layer 3 and is reflected as diffused light from the metallized layer 4, whereby the angle characteristic of reflex-reflection is deteriorated.
Since, however, the focus layer 3 is a thin layer of about 10-20 .mu.m which is normally formed by coating of a solvent-type coating, the surface of the coating material tends to become flat due to surface tension thereby making it extremely difficult to form the surface of the layer 3 contacting the metallized layer 4 into a partially spherical contour concentric with the glass beads.
Current Japan Industry Standard Z-9117 and U.S. Federal Standard LS-300C which are directed to the reflex-reflecting sheeting of the type described above allows the rate of decrease of about 50% for the angle of incidence of 30.degree. as compared with the angle of incidence of -4.degree..
It has been long since a reflex-reflecting sheeting of a type which is generally called "capsule type" was proposed for overcoming the disadvantage of the above described conventional type of reflex-reflecting sheeting.
The capsule type reflex-reflecting sheeting is of a structure in which, as shown in FIGS. 2 and 3, a metallized layer 4 is provided directly on the lower hemispheres of glass beads 2 which are disposed in spaced relation below a transparent protective film 1 with isolated hermetically sealed small cells 7 being formed between the glass beads 2 and the protective film 1, and a support film 5 underlying the metallized layer and having the lower hemispheres of the glass beads 2 embedded in the upper portion thereof and the protective film 1 overlying the glass beads 2 are vertically connected together by a connecting wall 6 which is formed in a continuous network in a plan view dividing the surface of the sheeting into hermetically sealed cells 7, 7 of small areas.
This structure obviates the transparent focus layer 3 shown in FIG. 1 and, accordingly, it is not necessary to maintain uniformity of thickness of the focus layer 3. This remarkably contributes to improving of the angle characteristic of the reflex-reflecting sheeting.
A typical example of the capsule type reflex-reflecting sheeting is disclosed in Japanese Patent Publication No. 7870/1965 (the specification of U.S. Pat. No. 3,190,178).
The structure of the reflex-reflecting sheeting and the method for producing the same disclosed in the above publication may be summarized with reference to FIG. 2 as follows: First of all, upper hemispheres of glass beads 2 are embedded provisionally in a support layer (not shown) and a metallized layer 4, 4' is vapor-coated over the lower hemispheres of the glass beads 2 and the surface of the support layer which is not occupied by the glass beads 2. Then support film 5 made of thermoplastic polymer is coated on the metallized layer 4, 4' and a heat-resistant film 8 is provided under the support film 5 to cover the lower surface thereof. The provisional support layer on the opposite side is stripped off and a biaxially-oriented transparent protective film 1 is laid over the upper hemispheres of the exposed glass beads 2. The laminate is heated and pressed from the side of the heat-resistant film 8 by a platen having a network pattern of raised ridges represented by FIG. 3 or of a lattice work for forming desired isolated small cells 7, 7. The support film 5 is partially melted to contact the transparent protective film 1 thereby forming the connecting wall 6 after the pattern of the platen which defines the isolated small cells 7.
While the structure in which the network connection between the protective film 1 and the support film 5 by the connecting wall 6 is formed by utilizing the support film 5 itself without using a separate bonding material is convenient, the material and mechanical structure of the support film 5 must have not only sufficient strength and flexibility but also properties necessary for an adhesive, i.e., sufficient cohesive force within the material itself and sufficient adhesive force relative to the protective film 1.
Selection of a suitable material for realizing such type of reflex-reflecting sheeting in a practicable form requires many tests and studies in addition to general knowledge concerning adhesives. The above described prior art, for example, selected the combination of thermoplastic polymethylmethacrylate as the support film 5 and biaxially oriented polymethylmethacrylate film as the protective film 1.
This prior art sheeting, however, has several drawbacks. One of them is that the connecting portions of the sheeting tends to be destroyed due to various external causes. The Japanese Preliminary Patent Publication No. 110592/1977 corresponding to U.S. Pat. No. 4,025,159 states in effect that the above described prior art reflex-reflecting sheeting which uses thermoplastic polymer is inadequate in its durability.
Although the Japanese Patent Publication No. 7870/1965 describes generally that hot-melt type thermosetting polymer may be used as the material for the support film, no specific example is given in the specification.
Secondly, the reflex-reflecting sheeting disclosed in Japanese Patent Publication No. 7870/1960 adopted such structure that, as shown in FIG. 2 of the Publication, the metallized layer 4, 4' covers the lower hemispheres of the glass beads 2 and the portion which is not occupied by the glass beads 2 in an uninterrupted layer. That is to say, the metal deposit constitutes an integral and continuous surface.
Since the area of the metallized layer 4' covering the portion which is not occupied by the glass beads 2 is considerably large in this structure, the reflecting sheeting appears dark.
For preventing light from reaching the upper surface of the metallized layer 4', a cover layer 9 of a bead-bond coating including a pigment such as a rutile type white pigment (TiO.sub.2) as shown in FIG. 2 must be provided. A part of this coating is present between the surfaces of the lower hemispheres of the glass beads 2 and the metallized layer 4 and this intervening coating tends to prevent the incident light from reaching the metallized layer through the glass beads thereby giving rise to the tendency that the rate of light reflex-reflection of the sheeting is reduced.
Japanese Preliminary Patent Publication No. 110592/1977 is a prior art directed to eliminate the above described drawback of the reflex-reflecting sheeting disclosed in Japanese Patent Publication No. 7870/1965, i.e., the inadequate durability due to utilization of a part of the support film of thermoplastic polymer as the connecting wall to the protective film.
Different from the general description in the Japanese Patent Publication No. 7870/1965, this publication teaches that the adhesion of the support film to the protective film can be remarkably improved by employing a specifically selected composition for the main material of the support film, i.e., a special composition prepared by mixing into a mixture of acrylic-based thermoplastic polymer similar to the one disclosed in Japanese Patent Publication No. 7870/1965 ingredients such as monomer polyethylene glycol diacrylate, 2-cyanoethyl acrylate and 1,6-hexanediol diacrylate which are polymerized and cured by irradiation of ultraviolet ray, electron beam or heat ray.
It would be convenient if the adhesion of the support film to the protective film had improved by the above described employment of the specific material. This proposed sheeting, however, has also several problems.
In other words, although the above known sheeting may have sufficient adhesion in the interface of the support film and the protective film, strength of the portion of the connecting wall for connecting the support film with the protective film is not much different from the sheeting in which the thermoplastic support film is employed and besides strength inside the connecting wall is not necessarily sufficient.
More specifically, as shown in FIG. 2, when peeling force is applied to the reflecting sheeting, breaking of the sheeting occurs not in the interface A of the support film material and the protective film but in the plane taken along line B-B' or the plane taken along line C-C' of the connecting wall 6.
This is considered to be only natural, when the fact that metal deposits and a considerable number of glass beads are included in the connecting wall 6 formed by a part of the material of the support film 5 is taken into consideration. In addition, it may be said that addition of a photosensitive monomer to the support film material tends to decrease breaking or peeling strength due to deterioration caused by the outdoor light.
The employment of a biaxially oriented film as the protective film in the respective patents seemingly is an advisable selection for increasing properties of the protective film including strength, permeability resistance and transparency.
It has, however, been confirmed by the inventor of the present invention that such selection actually is not advisable at all.
In fact, the reflecting sheetings of the above prior arts exhibit remarkable shrinkage when heated. More specifically, when the support film materials, i.e., bonding materials disclosed in Japanese Patent Publication No. 7870/1965 and Japanese Preliminary Patent Publication No. 110592/1977, are utilized, the reflecting sheetings shrink significantly in the condition that the sheetings are permitted to stand for 3 hours in an atmosphere at 93.degree. C. and then immersed in water for 21 hours, and such operation is repeated two or three times. Besides, at the higher temperature of 145.degree. C., the protective film 1 is curled in about only 1 or 2 minutes and peels from the remaining portion, resulting in complete destruction of the sheeting.
Moreover, the problem arising from the fact that a metallized layer exists over the support film in the portion which is not occupied by glass beads remains unsolved in the reflecting sheeting disclosed in Japanese Preliminary Patent Publication No. 110592/1977. This is because there is no structural difference between the Preliminary Patent Publication No. 110592/1977 and the first described prior art except that the binder, i.e., the support film 5 of a thermoplastic polymer in the prior art is merely replaced by one of a hot-melt type setting polymer.
Further, for manufacturing the reflecting sheeting employing the known setting type resin for the support film, it is required that the production line after partial thermoforming of the support film be provided with a special apparatus such as ultraviolet or heat ray irradiating apparatus, and the support film be irradiated for a certain period of time after thermoforming of the connecting wall.
For instance, in Examples 1-3, 5-10, and 12-14 of Japanese Preliminary Patent Publication No. 110592/1977, samples have been irradiated with a 190-kilovolt electron beam to a dose of 1.5 megarads by employing an electron beam irradiating apparatus, whilst in Example 4, the sheeting has been irradiated with ultraviolet rays using two passes at a rate of 50 ft./min. for curing the support film.
In Example 11, the support film has thermally been cured by heating for 16 hours at 65.degree. C. with the use of a heating device. In this respect, Preliminary Patent Publication No. 110592/1977 itself states in effect that such heat radiation for a long period of time is not desirable.
As described above, it is a significant disadvantage in manufacturing reflecting sheetings to require an operating step in which a special apparatus must be used for curing polymer.